Table 2.

Risk of ovarian carcinoma by breastfeeding duration and by subgroup

	Number of estimates	Pooled odds ratio and 95% confidence interval	p-value	I2 (%)
Breastfeeding category
Ever vs. Never	41	0.70 (0.64; 0.77)	<0.001	70.0
<6 months vs. Never	20	0.83 (0.78; 0.89)	<0.001	3.0
6–12 months vs. Never	19	0.72 (0.66; 0.78)	<0.001	22.0
>12 months vs. Never	29	0.63 (0.56; 0.71)	<0.001	51.8

Subgroup analysis (Ever vs. Never)	Number of estimates	Pooled odds ratio and 95% confidence interval	p-value	I2 (%)	Meta-regression p-value
Study size
<500 participants	7	0.67 (0.53; 0.84)	0.001	2.8	0.241
500–1499 participants	12	0.59 (0.47; 0.74)	<0.001	74.9
≥1500 participants	22	0.76 (0.69; 0.84)	<0.001	70.9
Setting
High income	35	0.74 (0.68; 0.80)	<0.001	64.0	0.007
Lower mid-income	6	0.48 (0.29; 0.77)	<0.001	77.1
Study design
Cohort	5	0.87 (0.78; 0.98)	0.02	0.0	0.136
Case–control	36	0.68 (0.61; 0.75)	<0.001	71.6
Mean age
≤49	10	0.70 (0.64; 0.77)	<0.001	70	0.744
>49	24	0.71 (0.63; 0.80)	<0.001	77
Adjusted for parity
Fine adjustment	16	0.80 (0.75; 0.86)	<0.001	56	0.231
Crude adjustment	4	0.69 (0.59; 0.81)	<0.001	45
No adjustment	21	0.67 (0.58; 0.77)	<0.001	72
Control for confounding
Thorough	14	0.76 (0.67; 0.85)	<0.001	64.9	0.419
Partial	15	0.66 (0.55; 0.81)	<0.001	73
None	11	0.72 (0.65; 0.78)	<0.001	50.5
Quality of study
Adequate	27	0.72 (0.65; 0.80)	<0.001	71	0.505
Inadequate	14	0.63 (0.58; 0.68)	<0.001	54.7